1. Field of Invention
This invention relates to a self-tuning controller which comprises a PI controlling means for effecting at least proportional integral (PI) arithmetic on a deviation signal between a process variable from a process and a control set value, to automatically adjust PI arithmetic parameters to an optimum value; and more particularly, to a self-tuning controller for adjusting the PI arithmetic parameters so that a response waveform exhibits an optimum response after observing the response waveform of the process variable or the deviation signal relative to disturbances created at random without the disturbances influencing the process.
2. Description of the Prior Art
In the prior art PI adjusting devices used with feedback control, the PI arithmetic parameters were manually set by a human operator based on his or her skills, technique, knowledge and experience. This is still the situation in the art. Manual setting,however, causes many disadvantages and defects. The process control is temporarily or constantly disturbed under such circumstances that start up of the process is affected, load fluctuates, and unexpected disturbances are intermixed, or under the condition of the system having non-linear gain properties. In some cases, an economical loss is brought about depending on the situation.
To resolve this problem, controllers for automatically tuning the PI arithmetic parameters have been proposed, but, none of them has been entirely satisfactory. Among them the main self-tuning controllers are as follows.
(i) A first type of controller is arranged such that an auxiliary controller is parallely connected to a main controller, gain of the auxiliary controller is increased to cause oscillation, and the PI arithmetic parameters are determined from the amplitude and frequency thereof on the basis of a so-called Z.multidot.N thereshold sensitivity method proposed by Ziegler and Nichols. This type of controller is disclosed in "A study of adaptive control system based on threshold sensitivity method", Toshiyuki Kitamori, appearing in A collection of Treatises of the Japan Measurement Automatic Control Association, 1970, pp55-60, Vol. 6.
(ii) A second type of controller is arranged such that a limit cycle is generated by use of an ON-OFF generator, and the optimum PI arithmetic parameters are determined from the amplitude thereof. This type of controller is disclosed in "PID automatic setting type adaptive controller" by Sumi and Fukuda, appearing in A collection of Preparatory Papers of the 12th Scientific Lecture of the Japan Automatic Control Association, pp617-624.
(iii) A third type controller is arranged such that pattern recognition means, i.e. performance measurement, for observing a behavior as a controlled variable is provided; disturbances produced at random in the control system are recognized without the disturbances influencing the process; the recognized result is compared with an optimum response model; and the optimum PI arithmetic parameters are determined so that the recongnized result of the pattern approximates the response model. This type of controller is disclosed in adaptive control Systems, Pergamon Press, 1963, pp 1-18.
(iv) A fourth type controller utilizes the technique described in paragraph (iii) and uses information on amount of overshoot, damping factor obtained from a peak value and a cycle in order to recognize the response pattern in connection with the disturbance in the control system. This controller is disclosed in U.S. Pat. No. 4,602,326.
Turning to FIG. 1, there is depicted, the controller which is shown in FIG. 1 of Page 2 of the book entitled "Adaptive Control Systems" described in paragraph (iii) hereinabove. This controller comprises observing, i.e. performance measurement, means 21 for observing a behavior or pattern associated with a process variable PV transmitted from a process 1 and a deviation signal by inputting a process variable PV, a control set value SV and a deviation signal between the process variable and the control set value; and an adaptive controller 22 for controlling the arithmetic parameters of a controller 2 by inputting evaluation indexes or figures of merit, which represent the patterns from the pattern observing means 21, with the result that this figure of merit exhibits an optimum response.
FIG. 2 illustrates the controller disclosed in U.S. Pat. No. 4,602,326 which fundamentally uses the technique described in FIG. 1, and comprises first comparator means 20 for obtaining a deviation between the process variable PV and the control set value SV; detecting means 21 for detecting characteristics of a pattern by observing the pattern of the process variable; second comparator means 22, coupled to the detecting means 21, for comparing a characteristic value of the detected pattern with a desired value of a preset pattern; and adjusting means 2, coupled to the second comparator means 22, for outputting a manipulated variable MV so that the process variable PV substantially accords with the control set value SV. Adjusting means 2 is constituted to halt action ,e.g. tuning, to modify the arithmetic parameters, if the difference between the characteristic value of the detected pattern and the desired value of the preset pattern is smaller than a predetermined value.
Detecting means 21 functions in such a manner that when the pattern, comprising a waveform of response signal, of the process variable varies in the manner depicted, for example, in FIG. 3., first, second and third peak values E.sub.1, E.sub.2 and E.sub.3 are detected; and an overshoot quantity, a damping factor and a cycle T.sub.p between the first peak and the third peak are obtained in order that these values serve as the characteristic values for the patterns. In this case, the overshoot quantity is given by -E.sub.2 /E.sub.1 while the damping factor is given by(E.sub.3 -E.sub.2)/(E.sub.1 -E.sub.2).
There arises, however, a problem inherent in the above prior art controllers. In the controllers mentioned in paragraphs (i) and (ii), it is required to forcibly impart disturbances (i.e. identifying signals) to the controlled system when the process is brought into oscillation, or when determining the arithmetic parameters. For this reason, the influences due to the disturbances are more or less exerted on the process.
The conventional controllers of FIGS. 1 and 2 do not adppt the method of forcibly imparting disturbances to the process system when determining the PI arithmetic parameters, or when putting the process into the oscillatory state. These controllers are superior in that disturbances do not influence the controlled system. The FIG. 2 controller uses one method of the FIG. 1 embodiment, but, disadvantageously, has the following problems.
(A) The detecting means 21 detects the first, second and third peak values E.sub.1, E.sub.2 and E.sub.3 of the response waveforms, which values are defined as the characteristic values of the patterns. Therefore, where the response waveforms are, as illustrated in FIGS. 4(A) and 4(B), similar to each other, the overshoot quantities and the damping values thereof are the same, if their amplitudes are different from each other. For this reason, when the amplitude of the waveform is small, the optimum arithmetic parameters are not tuned, thereby providing more unstable properties.
(B) The adjusting means 2 is arranged to stop tuning the arithmetic parameters if the difference between the characteristic value of the detected pattern and the desired value of the preset pattern decreases below the predetermined value, and hence it takes a substantial amount of time to pursue the arithmetic parameters up to the area of the optimum values.